The present invention relates generally to gamma counters, i.e., instruments for the detection and measurement of gamma radiation. More particularly, the invention relates to a novel calibration technique for use with such instruments.
Typically, a gamma counter includes a radiation detector in the form of a sodium iodide crystal activated with thallium. Gamma rays emitted from a radioactive sample being monitored excite some of the electrons in the sodium iodide, and the excited electrons react with the thallium to produce light scintillations. The scintillations are then detected by a multiplier phototube and converted into corresponding electrical pulses. The resultant output pulses from the multiplier phototube should be directly proportional, in amplitude, to the energies of corresponding gamma rays from which the pulses were derived. Gamma counters usually include some means for sorting or filtering the output pulses from the phototube, so that an energy or pulse-height spectrum can be obtained.
By way of background, it should be noted that the energy spectrum that can be obtained by use of a gamma counter does not accurately reflect the energy spectrum of the incident radiation. Gamma rays are essentially monoenergetic, i.e., if a radioactive substance has the characteristic that it emits gamma rays at a particular energy level, every gamma ray from the substance will be emitted at exactly the same energy level. If an energy spectrum relating to the gamma radiation were to be plotted, with a count of detected gamma rays plotted along the vertical axis and the gamma ray energy plotted along the horizontal axis, the resulting spectrum would be a vertical line located at the energy level corresponding to the gamma radiation from the radioactive substance or would be a number of such vertical lines, if the substance emits gamma radiation at a number of different energy levels. In practice, however, such a spectrum can never be obtained from a gamma counter. The sodium iodide scintillator does not always generate exactly the same number of excited electrons from each incident gamma ray, and the multiplier phototube does not always produce exactly the same amplification each time a scintillation is detected by its photocathode. Consequently, the energy or pulse-height spectrum relating to output from the multiplier phototube will consist of a bell-shaped gaussian distribution, rather than a vertical line in the spectrum corresponding to the energy level of the incident gamma rays. This distribution is usually referred to as a photopeak in the pulse-height spectrum.
Most gamma counters include one or more pulse-height analyzers connected to receive output pulses from the multiplier phototube. Each pulse-height analyzer has upper and lower discriminator limits or settings which can be adjusted to define a desired "window" in the pulse-height spectrum. The pulse-height analyzer acts essentially as a filter, rejecting pulses which fall outside of the selected discriminator settings, and passing pulses which fall within the window to a scaler or counting device. The discriminator settings on a pulse-height analyzer are usually defined over an arbitrary scale, for example, between 0 and 1000. For many applications of a gamma counter, however, it is important to be able to obtain the results in terms of absolute energy levels. It is important, then, to be able to calibrate the gamma counter so that the relative range of the discriminator settings, from 0 to 1000, can be related to an absolute energy range, usually measured in millions of electron volts (MeV) or thousands of electron volts (KeV).
Ideally, a gamma counter should have a number of different energy ranges over which gamma radiation may be measured. For example, the relative scale of 0 to 1000 may correspond to an energy range of 0-0.5 MeV for one particular test, but may correspond to energy ranges of 0-1.0 MeV or 0-2.0 MeV for other tests using the same instrument. Prior to this invention, gamma counters have been calibrated for one particular energy range, usually the lowest energy range, then other energy ranges were obtained by the use of precision attenuators to reduce the pulse heights by an appropriate factor. For example, a gamma counter could be calibrated for a 0-0.5 MeV range by measuring the radiation from a calibration source known to emit gamma rays at a 0.25 MeV energy level. The attenuators would then be adjusted until the photopeak resulting from radiation from the calibration source was aligned with the 500th discriminator level setting, i.e., exactly of half-scale. The full-scale reading would then be 0.5 MeV, as desired. To obtain an energy range of 0-1.0 MeV, the attenuators would be adjusted to provide an additional attenuation factor of two, thereby halving the amplitude of all of the output pulses and providing an absolute energy range of 0-1.0 MeV. Similarly, to provide an absolute energy range of 0-2.0 MeV, a further attenuation factor of two would be interposed, again halving the amplitude of all of the peaks output from the multiplier phototube, and providing the desired energy range. The multiplier phototube would, in all instances, be operated at or near its maximum operating voltage.
It will be appreciated that any error in the calibration of the lowest range, 0-0.5 MeV in the foregoing example, will be magnified when the instrument is used on its higher ranges. Accordingly, there is a need in the field of gamma radiation measurement for a calibration technique which provides for the accurate calibration of the instrument for a number of different energy ranges, wherein the calibration at any one energy range is independent of the calibrations of the other energy ranges. The present invention fulfills this need.